Dwarfs .... Cool fish, although that gravel is rather nasty imho! lol, and yes, you need to get some algae scraper action going on with that glass!
Glolite ..... most plecs do absolutely fine with DPs... I have a BN plec with mine at the moment as it had been naughty and had a fight with my Syno Angelicus so i had to move him.
Remember, plecs are armoured catfish, they are pretty hardy against attack, so they wont take much from the DPs..
40L would house 3-4 DPs and a plec as long as the filtration is ok and weekly water changes done.
Ben